Disposable hard mask for photomask plasma etching

ABSTRACT

A method for creating a photomask which includes a layer of hard mask material the inclusion of which improves the uniformity of critical dimensions on the photomask by minimizing the affect of macro and micro loading. The method for producing the photomask of the instant invention includes two etching processes. The first etching process etches the layer of hard mask, and the second etching process etches the anti-reflective material and opaque material.

CROSS-REFERENCE TO RELATED APPLICATION

This patent application is a continuation of U.S. application Ser. No.09/409,454, filed Sep. 30, 1999 now U.S. Pat. No. 6,472,107, andentitled “DISPOSABLE HARD MASK FOR PHOTOMASK PLASMA ETCHING,” thecontents of which are incorporated herein by reference in theirentirety.

BACKGROUND

The present invention relates to a photomask which includes a hard masklayer, the use of which improves the uniformity of critical dimensionson the photomask.

Photomasks are used in the semiconductor industry to transfermicro-scale images defining a semiconductor circuit onto a silicon orgallium arsenide substrate or wafer. A typical binary photomask iscomprised of a transparent quartz substrate and chrome (Cr) opaquematerial that includes an integral layer of chrome oxide (CrO)anti-reflective (AR) material. The pattern of the Cr opaque material andCrO AR material on the quartz substrate is a scaled negative of theimage desired to be formed on the semiconductor wafer.

To create an image on a semiconductor wafer, a photomask is interposedbetween the semiconductor wafer, which includes a layer ofphotosensitive material, and an energy source commonly referred to as aStepper. The energy generated by the Stepper passes through thetransparent portions of the quartz substrate not covered by the Cropaque material and the CrO AR material, and causes a reaction in thephotosensitive material on the semiconductor wafer. Energy from theStepper is inhibited from passing through the areas of the photomask inwhich the Cr opaque material and CrO AR is present. The CrO AR materialprevents most, but not all, of the incident energy from being reflectedback into the Stepper. If excess energy is reflected back into theStepper a degraded image will be created in the photosensitive resistmaterial on the semiconductor wafer surface, thereby resulting in adegradation of performance of the semiconductor device.

A finished photomask used in the production of semiconductor devices isformed from a “blank” or “undeveloped” photomask. As shown in FIG. 1, aprior art blank photomask 20 is comprised of four layers. The firstlayer 2 is a layer of quartz, commonly referred to as the substrate, andis typically approximately one quarter inch thick. Affixed to the quartzsubstrate 2 is a layer of Cr opaque material 4 which typically isapproximately 900 Å to 1000 Å thick. An integral layer of CrO ARmaterial 6 is formed on top of the layer of Cr opaque material 4. Thelayer of CrO AR material is typically approximately 100 Å thick. A layerof photosensitive resist material 8 resides on top of the CrO ARmaterial 6. The photosensitive resist material 8 is typically ahydrocarbon polymer, the various compositions and thicknesses of whichare well known in the art.

The desired pattern of Cr opaque material to be created on the photomaskmay be defined by an electronic data file loaded into an exposure systemwhich typically scans an electron beam (E-beam) or laser beam in araster fashion across the blank photomask. One such example of a rasterscan exposure system is described in U.S. Pat. No. 3,900,737 to Collier.As the E-beam or laser beam is scanned across the blank photomask, theexposure system directs the E-beam or laser beam at addressablelocations on the photomask as defined by the electronic data file. Theareas of the photosensitive resist material that are exposed to theE-beam or laser beam become soluble while the unexposed portions remaininsoluble. As shown in FIG. 2, after the exposure system has scanned thedesired image onto the photosensitive resist material, the solublephotosensitive resist is removed by means well known in the art, and theunexposed, insoluble photosensitive resist material 10 remains adheredto the CrO AR material 6.

As illustrated in FIG. 3, the exposed CrO AR material and the underlyingCr opaque material no longer covered by the photosensitive resistmaterial in the prior art phoromask 22 is removed by a well knownetching process, and only the portions of CrO AR material 12 and Cropaque material 14 residing beneath the remaining photosensitive resistmaterial 10 remain affixed to quartz substrate 2. This initial or baseetching may be accomplished by either a wet-etching or dry-etchingprocess both of which are well known in the art. In general, wet-etchingprocess uses a liquid acid solution to erode away the exposed CrO ARmaterial and Cr opaque material. A dry-etching process, also referred toas plasma etching, utilizes electrified gases, typically a mixture ofchlorine and oxygen, to remove the exposed chrome oxide AR material andchrome opaque material.

A dry-etching process is conducted in vacuum chamber in which gases,typically chlorine and oxygen are injected. An electrical field iscreated between an anode and a cathode in the vacuum chamber therebyforming a reactive gas plasma. Positive ions of the reactive gas plasmaare accelerated toward the photomask which is oriented such that thesurface area of the quartz substrate is perpendicular to the electricalfield. The directional ion bombardment enhances the etch rate of the Cropaque material and CrO AR material in the vertical direction but not inthe horizontal direction (i.e., the etching is anisotropic ordirectional).

The reaction between the reactive gas plasma and the Cr opaque materialand CrO AR material is a two step process. First, a reaction between thechlorine gas and exposed CrO AR material and Cr opaque material formschrome radical species. The oxygen then reacts with the chrome radicalspecies to create a volatile which can “boil off” thereby removing theexposed CrO AR material and the exposed Cr opaque material.

As shown in FIG. 4, after the etching process is completed thephotosensitive resist material in the prior art photomask 24 is strippedaway by a process well known in the art. The dimensions of the Cr opaquematerial on the finished photomask 26 are then measured to determinewhether or not critical dimensions are within specified tolerances.Critical dimensions may be measured at a number of locations on thefinished photomask, summed, and then divided by the number ofmeasurements to obtain a numerical average of the critical dimensions.This obtained average is then compared to a specified target number(i.e., a mean to target comparison) to ensure compliance with predefinedcritical dimensions specifications. Additionally, it is desired thatthere is a small variance among the critical dimensions on thesubstrate. Accordingly, the measured critical dimensions typically mustalso conform to a specified uniformity requirement. Uniformity istypically defined as a range (maximum minus minimum) or a standarddeviation of a population of measurements.

The etch rate of the plasma etching process described above (and hencethe uniformity of the critical dimensions) is dependent on the desiredpattern to be formed in the Cr opaque material and CrO AR material. Inareas of the photomask where a substantial portion of Cr opaque materialand CrO AR material are to be removed (i.e., macro loading), the etchingprocess may take longer than in areas of the photomask in which smallportions of Cr opaque material and CrO AR material are to be removed.Likewise, there may be differences in etch rate for micro loadingconditions in which the etch rate is different between isolated anddense features in the same general area. These differing etch rates makeit more difficult for the finished photomask to conform to a specifieduniformity requirement. Additionally, the above described etchingprocess can also cause variances in critical dimensions because thephotosensitive resist material is not entirely impervious to the plasmagases.

SUMMARY OF INVENTION

Accordingly, it is an object of the present invention to provide a blankphotomask which includes a layer of hard mask material thereby enablingthe critical dimensions of a finished photomask to be more uniform.

It is a further object of the invention to provide a method formanufacturing a finished photomask having improved uniformity ofcritical dimensions.

It is still further an object of the present invention to provide afinished photomask having improved uniformity in critical dimensions andimproved anti-reflection properties thereby reducing the amount of errorintroduced by the basic lithography process.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view of a blank photomask illustrating thecomposition of the various layers of a typical prior art blank binaryphotomask.

FIG. 2 is a cross-sectional view of a prior art photomask after exposureto an energy source and having the soluble photosensitive materialremoved.

FIG. 3 is a cross-sectional view of a prior art binary photomask afterbeing subjected to an etching process thereby removing the exposed CrOAR material and Cr opaque material.

FIG. 4 is a cross-sectional view of a finished, prior art binaryphotomask with the photosensitive material stripped away.

FIG. 5 is a cross-sectional view of a blank photomask is accordance withthe instant invention illustrating the composition of the various layersof a typical blank photomask including a hard mask layer.

FIG. 6 is a cross-sectional view of a photomask in accordance with theinstant invention after exposure to an energy source and having thesoluble photosensitive material removed.

FIG. 7 is a cross-sectional view of a photomask in accordance with theinstant invention after being subjected to a first etching processthereby removing the exposed hard mask material.

FIG. 8 is a cross-sectional view of a finished photomask in accordancewith, the first embodiment of the invention after being subjected to asecond etching process thereby removing the exposed CrO AR material andCr opaque material.

FIG. 9 is cross-sectional view of a second embodiment of a finishedphotomask is accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 5 depicts a blank photomask in accordance with the presentinvention. As shown the blank photomask 30 is comprised of a quartzsubstrate 2 having a thickness of approximately one quarter inch.Affixed to quartz substrate 2 is a layer of Cr opaque material 4 whichis typically 900 Å to 1000 Å thick. An integral layer of CrO AR material6, typically approximately 100 Å thick, is formed on top of the Cropaque material 4. Hard mask layer 18 is deposited on top of the CrO ARmaterial 6. The hard mask layer 18 may be deposited on the CrO AR layerusing a sputtering process or any other method well known in the artsuch as chemical vapor deposition (CVD). The thickness of the hard masklayer is preferably in the range of 50 to 500 Å, and is most preferablyapproximately 250 Å thick. In the preferred embodiment, the hard masklayer is comprised of silicon (Si). However, the hard mask layer mayalso be comprised of other similar materials including but not limitedto Ti, TiW, W, TiN, Si₃N₄, SiO₂, or spin-on-glass.

The process for creating a finished photomask, having improved criticaldimensions and anti-reflective properties, from blank photomask 30 isnow described with reference to FIGS. 5 through 9. Initially, thedesired pattern to be formed in the hard mask material 18, the CrO ARmaterial 6, and the Cr opaque material 4 is scanned onto the layer ofphotosensitive resist material 8 of blank photomask 30 by means of araster scan exposure system, or comparable system (e.g., a vector scantool). The portions of the photosensitive resist material that areexposed to the E-beam or laser beam become soluble while the unexposedportions remain insoluble. As shown in FIG. 6, after the exposure systemhas scanned the desired image onto the photosensitive resist material,the soluble photosensitive resist is removed by means well known in theart exposing areas of hard mask material 18, and the unexposed,insoluble photosensitive resist material 10 remains adhered to, andcovering, other areas of the hard mask material 18.

Photomask 32 of FIG. 6 is next subjected to a first etching process toremove the exposed hard mask material. In the preferred embodiment wherethe hard mask is comprised of Si, the composition of the plasma gas haseither a fluorine, chlorine, or bromine containing species or acombination of various halide containing species such as, but notlimited to, C₂F₆, CHF₃, Cl₂, HBr, or SF₆. In the preferred embodiment,Cl₂ plasma gas is used to etch the Si hard mask. This is advantageoussince the same chamber can be used to etch both the hard mask and the Crand CrO materials without the need for any additional gas hookups. TheC₂F₆, CHF₃, HBr, Cl₂, or SF₆ plasma gases etch the exposed Si hard maskmaterial in a similar manner as described above with respect to theplasma etching of the Cr and CrO AR material. Silicon halides arevolatile and they will be readily removed once formed. However the C₂F₆,CHF₃, HBr, Cl₂, or SF₆ plasma gases do not significantly react with theunderlying Cr and CrO AR material. Thus, as shown in FIG. 7, only theportions of the exposed hard mask material 18 are removed by the firstetching and those portions of the hard mask material underlying thephotosensitive resist material 10 are not significantly affected. Asshown in FIG. 7, the first etching process exposes those areas of the Cropaque material and CrO AR material not underlying photosensitive resistmaterial 10 and hard mask material 18.

Photomask 34 of FIG. 7 is next subjected to a second etching process toremove the Cr and CrO layers. The etching process may be conducted withthe photosensitive resist material in place, or the photosensitiveresist material can be stripped away prior to commencement of the secondetching process. As described above, the second etching process isconducted in vacuum chamber in which chlorine and oxygen gases areinjected. An electrical field is created between and anode and a cathodein the vacuum chamber thereby forming a reactive gas plasma, andpositive ions of the reactive gas plasma are accelerated towardphotomask, which is at the same potential as the cathode, and which isoriented such that the surface area of quartz substrate is perpendicularto the electrical field. The reaction between the reactive gas plasmaand the Cr opaque material and CrO AR material is a two step process.First, a reaction between the chlorine gas and exposed CrO AR materialand Cr opaque material forms chrome radical species. The oxygen thenreacts with the chrome radical species to create a volatile which can“boil off” thereby removing the exposed CrO AR material and the exposedCr opaque material.

It will be appreciated by those skilled in the art that throughmodification of the chlorine to oxygen ratio, one can retain the hardmask material through the second etch process step. Accordingly, thesecond etching process can be extended to overcome the effects of macroloading. In other words, the effect of the differing etch rates in areasof photomask 34 having large portions of Cr material and CrO AR materialto be removed verses those areas in which only small portions of Cr andCrO AR materials are removed, can be eliminated. Additionally, withprior art photomasks which do not have a hard mask layer, the durationof the etching process is time critical due to the effects of the plasmagases on the photosensitive resist material. That is, as the Cr and CrOis being etched away, the plasma gases are also reacting with thephotosensitive material, and if the etching process is continued for toolong a period, the photosenstive material may no longer protect theunderlying portions of the Cr and CrO AR material from the plasma gases.In short, unlike the etching of prior art photomasks which do notinclude a layer of hard mask material, the second etching process can beextended in time to ensure that essentially all the exposed Cr and CrOAR materials are removed.

As shown in FIG. 8, the second etching process results in the exposureof only those portions of the quartz substrate 2 which correspond to thepattern originally scanned into the photosensitive material. Aftercompletion of the photosensitive material 10 may be stripped away by aprocess well known in the art, if not already done so prior to thecommencement of the second etching process. As shown in FIG. 8, theresulting photomask 36 of a first embodiment of the instant inventionhaving improved uniformity of critical dimensions is comprised of aquartz substrate 2 and patterned layers of Cr opaque material 4, CrO ARmaterial 6, and hard mask material 18. In this first embodiment of theinstant invention, hard mask material 18 remains an integral part of thedeliverable photomask. This embodiment of the invention is advantageousin that the hard mask material has a de minimis thickness but, dependingon its composition, may exhibit excellent anti-reflectivecharacteristics. Hard mask materials of Ti, TiN, TiW, W, and Si exhibitgood anti-reflective properties, while hard mask materials of Si₃N₄,spin-on-glass, and SiO₂ do not exhibit anti-reflective properties.

Alternatively, in a second embodiment of the instant invention shown inFIG. 9, the hard mask 18 can be stripped away using wet or dry etchingmethods. For example, an aqueous KOH solution can be used to strip awaythe silicon hard mask.

Although the photomask 38 of FIG. 9 appears substantially identical tothe prior art photomask shown in FIG. 4, those skilled in the art willappreciate that the critical dimensions of photomask 38 made inaccordance with the instant invention will have improved uniformity incritical dimensions.

Although the instant invention has been described with respect to theparticular embodiment of typical binary masks being comprised of Cr andCrO materials, those skilled in the art will appreciate that the instantinvention can be used with photomask of different types including PhaseShift masks (PSM) and Next Generation Lithography (NGL) masks where theinvented hard mask approach will benefit the manufacturability of thesemasks.

Additionally, depending on the composition of the opaque material,anti-reflective material, and hard mask material, different plasma gasesmay be used in the first and second etching processes. For example,chlorine may be used to etch a Si hard mask. A high oxygen concentrationmixture of oxygen and chlorine may be used to perform the second etchingfor Cr. If SiO₂ is used as the hard mask, fluorinated species may beused to etch the hard mask.

Accordingly, the spirit and scope of the instant invention is to beconstrued broadly and limited only be the appended claims, and not bythe foregoing specification.

What is claimed is:
 1. A method for creating an image on a blank photomask comprising a photosensitive resist material layer, a hard mask layer underlying said photosensitive resist material layer, said hard mask layer made from materials which are selectively resistant to etching in said blank photomask, an opaque layer underlying said hard mask layer, and a substantially transparent substrate layer underlying said opaque layer, said method for creating an image comprising the steps of: creating a patterned image in said photosensitive resist layer; removing portions of said photosensitive resist layer that do not correspond to said patterned image thereby exposing portions of said hard mask layer not corresponding to said patterned image; removing said exposed portions of said hard mask layer that do not correspond to said patterned image thereby exposing portions of said opaque layer not corresponding to said patterned image; removing said exposed portions of said opaque layer underlying said hard mask layer portions that do not correspond to said patterned image thereby exposing portions of said substantially transparent layer that do not correspond to said patterned image; and removing said photosensitive resist layer.
 2. The method of claim 1, wherein said hard mask layer is comprised of Si and said step of removing said exposed portions of said hard mask layer is conducted using plasma gases, the composition of which has either fluorine, chlorine, or bromine containing species or a combination of various halide containing species.
 3. The method of claim 1, wherein said step of removing said exposed portions of said hard mask layer is conducted using plasma gases selected from the group consisting of C₂F₆, CHF₃, HBr, Cl₂, and SF₆.
 4. The method of claim 1, wherein said hard mask layer is selected from the group consisting of Ti, TiW, W, Si₃N₄, SiO₂, TIN, and spin-on-glass and said step of removing said exposed portions of said hard mask layer is conducted using plasma gases, the composition of which has either fluorine, chlorine, or bromine containing species or a combination of various halide containing species.
 5. The method of claim 4, wherein said step of removing said exposed portions of said hard mask layer is conducted using plasma gases selected from the group consisting of C₂F₆, CHF₃, HBr, Cl₂, and SF₆.
 6. A method for creating an image on a photomask comprising a photosensitive resist material layer, a hard mask layer underlying said photosensitive resist material layer, said hard mask layer made from materials which are selectively resistant to etching in said photomask, an anti-reflective layer underlying said hard mask layer, an opaque layer underlying said anti-reflective layer, and a substantially transparent substrate layer underlying said opaque layer, said method for creating an image comprising the steps of: creating a patterned image in said photosensitive resist layer; removing portions of said photosensitive resist layer that do not correspond to said patterned image thereby exposing portions of said hard mask layer not corresponding to said patterned image; removing said exposed portions of said hard mask layer that do not correspond to said patterned image thereby exposing portions of said anti-reflective layer overlying portions of said opaque layer not corresponding to said patterned image; removing said exposed portions of said opaque layer underlying said anti-reflective portions that do not correspond to said patterned image thereby exposing portions of said substantially transparent layer that do not correspond to said patterned image; and removing said photosensitive resist layer.
 7. The method of claim 6, wherein said hard mask layer is comprised of Si and said step of removing said exposed portions of said hard mask layer is conducted using plasma gases, the composition of which has either fluorine, chlorine, or bromine containing species or a combination of various halide containing species.
 8. The method of claim 6, wherein said step of removing said exposed portions of said hard mask layer is conducted using plasma gases selected from the group consisting of C₂F₆, CHF₃, HBr, Cl₂, and SF₆.
 9. The method of claim 6, wherein said hard mask layer is selected from the group consisting of Ti, TiW, W, Si₃N₄, SiO₂, TiN, and spin-on-glass and said step of removing said exposed portions of said hard mask layer is conducted using plasma gases, the composition of which has either fluorine, chlorine, or bromine containing species or a combination of various halide containing species.
 10. The method of claim 9, wherein said step of removing said exposed portions of said hard mask layer is conducted using plasma gases selected from the group consisting of C₂F₆, CHF₃, HBr, Cl₂, and SF₆.
 11. The method of claim 6, further comprising removing said exposed portions of said anti-reflective layer and said portions of said opaque layer that do not correspond to said patterned image by using plasma gases comprised of Cl₂ and O₂ thereby exposing portions of said substantially transparent substrate layer not corresponding to said patterned image, wherein said opaque layer is comprised of Cr and said anti-reflective layer is comprised of CrO. 